The goals of this project are to: (1) fully characterize SIRD as a distinct form of organ failure impairing O2 delivery in sepsis, (2) elucidate SIRD's role in multiple organ failure (MOF) progression, and (3) evaluate a mechanism-based therapy targeted to SIRD pathobiology. In sepsis, a number of RBC defects have been (individually) described: altered O2 affinity, membrane deformability, RBC aggregation and adhesion, as well as dysregulated RBC-based nitric oxide (NO) processing. We suggest that these defects comprise a unique class of organ failure (which we term SiRD) that disables transport of O2 from lungs to tissue. Based upon our preliminary findings, we propose the novel hypothesis that in sepsis, energetic support of RBC antioxidant systems fails, with SiRD arising consequent to unquenched reactive oxygen species (ROS) generated in the course of hemoglobin O2 binding/release. As such, by critically impairing O2 delivery (by limiting both delivery of RBCs to tissue [e.g. flow] and release of O2 from delivered RBCs), SiRD exacerbates dysoxia and MOF progression. We propose a mechanistic 'reverse translational' approach to test this hypothesis in a comprehensively phenotyped cohort of children with severe sepsis (enabling us to study subjects lacking comorbidities which also impair RBC function, e.g. diabetes, renal failure, etc.). We will study children in the Inflammation Phenotypes in Pediatric Sepsis Induced Multiple Organ Failure (PHENOMS Trial [GM108618], which will be conducted by the NICHD Collaborative Pediatric Critical Care Research Network [CPCCRN]). This will enable us to leverage the established CPCCRN infrastructure and the detailed phenotype and outcome evaluation of the PHENOMS cohort so that we may link SiRD to progression of sepsis syndromes, MOF evolution, and to outcome. We will structure our approach by pursuing the following Specific Aims: SA1. Define sepsis-induced biochemical alterations to RBCs that influence O2 delivery. PHENOMS subjects' RBCs will be studied in ex vivo assay platforms, organ bioassays and in vivo models to quantitate defects in (and efficacy of SOD mimetics in restoring): O2 binding/delivery as well as control of vascular tone and blood flow. SA2 Define sepsis-induced biophysical alterations to RBCs that influence O2 delivery. As above, subjects' RBCs will be studied employing state of the art biophysical analysis (for membrane deformability, RBC aggregation and endothelial adhesion) and intravital microscopy to quantitate defects in (and efficacy of SOD mimetics in restoring): RBC transit through vascular channels and adhesion to activated endothelium. SA3 Characterize sepsis-induced alterations in RBC energy metabolism, antioxidant systems and oxidative injury. Study subjects' RBCs will be subjected to controlled oxidative loading to quantitate the dynamic range in (and efficacy of SOD mimetics in restoring): glycolytic flux (1H NMR analysis of lactate isotopomers), redox poise in antioxidant systems, and (c) oxidative injury to membrane and proteins.